This invention relates to an improved method for curing epoxy resin and for making an amine epoxy curing agent that enables the mixture of epoxy resin and its curing agent to be coated on underwater substrates and cured there and also to be coated on wet and damp surfaces and cured there.
Most amine epoxy curing agents are water soluble or at least partly water soluble. Their water solubility renders these prior-art curing agents ineffective for adequate cure of epoxy resins when they are either coated underwater over a substrate or on wet surfaces, because when such cure is attempted, the curing agent partially dissloves in water. As a result, the mixture tends to slip around on the surface and not adhere. In addition, the ratio of epoxy to curing agent has to be altered considerably from the optimum, to accommodate an unknown amount of curing agent that will dissolve in water, and an additional amount of curing agent will therefor be needed. Even then, the results are generally unacceptable.
A good example of curing agents that are partially water soluble are the common amine-amide-dimer acid curing agents known as "polyamide curing agents"; an example of this is Versamid 125 made by Henkel. This material forms a soft jelly-like mass when mixed with water, due to its partial water solubility. When one attempts to coat a mixture of epoxy resin and Versamid 125 under water or on wet surfaces, the coating slips around on the wet surface and cannot make a satisfactory film.
Satisfactory underwater coating, as well as coating on wet or moist surfaces, can be obtained by using the invention described and claimed in my earlier U.S. Pat. No. 4,022,946. The curing agents set forth in that patent are water insoluble or substantially so. Two types are set forth:
(1) amines of the formula ##STR1## in which R is chosen from the group consisting of ether radicals of which one of the alkyl or alkylene radicals has at least eight carbon atoms, and
(2) a condensation product of a small proportion of a polyepoxide having a plurality of vic-epoxy groups and containing only members of the groups consisting of carbon, hydrogen, oxygen, and chlorine, and a larger proportion of the amine of (1) above.
However, the product of U.S. Pat. No. 4,022,946 has many drawbacks. It shows erratic cure, often poor, under ordinary conditions and cures very slowly under cold conditions. The cured film is often soft and low in strength; it exhibits poor chemical resistance and poor resistance to solvents. Its application requires the use of a diluent, because the uncured mixture is very viscous. Propylene glycol, the usual diluent, is approved for use in food handling areas, but it detracts from the desired water insolubility. Finally, the product tends to be expensive, partly because it is usually used in its adduct form, and the adducted epoxy resin is expensive.
Among the objects of the present invention are: the provision of an improved epoxy-resin curing agent having improved water insolubility, and imparting improved water insolubility to the mixture of the resin and curing agent; the provision of an improved agent for curing epoxy resins under water and for adhering them to various substrates, both in moist conditions and under water; the provision of improved cure under ordinary conditions, with more reliability and predictability and better resin adherence; the provision of a product that cures epoxy resins more quickly than comparable products under cold conditions; the provision of a curing agent that imparts to the epoxy resin improved toughness and flexibility even when the curing is done under water, the cured film also having excellent chemical resistance and excellent solvent resistance; the provision of a curing agent requiring no diluents because it has in itself a sufficiently low viscosity, which is controllable; and the provision of a curing agent, usable with epoxy resins under wet or moist conditions, that is less expensive than comparable curing agents.